The present invention relates to a method of imaging an object or substance using an ultrasonic wave non-destructive inspection device which detects in the metal material, for example, to display the image of the defects in high resolution in real time, or a method of imaging an object by synthetic aperture radar capable of imaging ground surface conditions by remote operation in the air by the electromagnetic waves.
The manner employed for prior-art ultrasonic wave non-destructive inspection includes measuring the propagation time from the transmission to the reception of the reflected signal of the space information at some point of the object to be imaged by focusing the ultrasonic wave beam, electronically or mechanically scanning sequentially an ultrasonic transceiver element, and imaging and displaying the object to be imaged as a compilation of the point information.
A prior-art apparatus for this type of system is simple in itself, but the resolution or the azimuth resolution in the scanning direction, depends upon the focusing degree of the ultrasonic wave beam, that is, the azimuth resolution is provided by the spread of the beam itself, and the prior-art apparatus has a defect in that the azimuth resolution gets worse in proportion to the distance to the object since the spread of the beam is proportional to the distance to the object. Thus, particularly in recent times, this apparatus has not always sufficiently answered the requests for quantizing the shape of defects in the material so as to evaluate the healthy or remaining lifetime of the structural material for the piping welded portions of the recent atomic or thermal power plants.
Ultrasonic wave non-destructive inspection using a synthetic aperture method is intended to remove such disadvantages of the above mentioned pulse echo method, and it has features for improving the azimuth resolution and for obtaining a predetermined azimuth resolution irrespective of the distance to the object to be imaged. This will be described with reference to FIGS. 1 and 2. In FIG. 1, reference numeral 1 denotes an ultrasonic wave transceiver element having aperture d and capable of transmitting and receiving the ultrasonic wave, numeral 2 denotes an ultrasonic wave beam having a beam expanding angle .theta..omega. transmitted from the transceiver element 1, and numeral 3 denotes an object to be imaged, which is here taken to be a point object. Numeral 4 denotes a propagating medium interposed between the transceiver element 1 and the object 3, and numeral 5 denotes the scanning line (plane) of the transceiver element 1. Reference symbol f denotes the central frequency of the transmitted ultrasonic wave from the transceiver element 1, symbol C denotes the sonic velocity in the medium 4, and symbol L denotes the length of the scanning range of the transceiver 1 capable of observing the object 3 by the beam 2. In case the scanning direction of the transceiver element 1 is in the x-axis and the depthwise direction perpendicularly crossing the x-axis is in the z-axis, the object 3 is disposed at the point (Xo, Zo) in the x-z plane, the transceiver element scans on the scanning line 5 while transmitting and receiving the ultrasonic wave, and it is disposed at the point (x, o). FIG. 2 shows the relationship between the reception signal of the transceiver element 1 resulting from the reflection from the object 3 to be imaged at each scanning point (transmitting point) in FIG. 1 and the time from the point of transmittal. Here, the time t(x) from the transmission o the reception of the received ultrasonic wave signal of the transceiver element 1 at the scanning point (x, o), i.e., the phase delay is given by the following equation (1). ##EQU1## The time of flight locus (hereinafter referred to as "TOF locus") given by the equation (1) forms the hyperbolic curve as shown in FIG. 2 by the broken line. The signal intensity resulting from the object reflection dispersed in time space on the hyperbolic curve in FIG. 2 is said to be compressable on the corresponding object point of the object 3 to be imaged by coherent addition (same phase) of the received signal within the range of length L. This is physically equivalent to the sequential occupation of the aperture of the ultrasonic transceiver element having an aperture of length L determined from the spreading angle .theta..omega. of the ultrasonic wave beam 2 by the scanning points on the scanning line 5 in FIG. 1, i.e., the emission of the object 3 to be imaged by the transceiver element having aperture L. This length L is called "synthetic aperture length", and the method of forming an image of the object 3 to be imaged in this manner is called "synthetic aperture method".
In this case, the azimuth resolution .delta.x becomes as follows in the following equation (2): EQU .delta.x=(.lambda./L)Zo (2)
where .lambda. represents the ultrasonic wave wavelength. Symbol L is given by the spread .lambda./d of the ultrasonic wave beam, and the distance Z.sub.o to the object 3 to be imaged as represented by the following equation (3). EQU L=(.lambda./d) Zo (3)
The L determined by the equation (3) is substituted for the equation (2), and the azimuth resolution is eventually represented as the following equation (4). EQU .delta.x=d (4)
The azimuth resolution by the synthetic aperture method does not depend upon the distance Z.sub.o to the object 3 to be imaged as identified from the equation (4) but it becomes constant in the degree of the aperture d of the transceiver element 1.
The execution of the object imaging method by the synthetic aperture method, for example, with respect to the x-z plane in FIG. 1 will be described with reference to FIG. 3. In FIG. 3, the area reproduced by the received signal group at all the scanning points of the range of the synthetic aperture length L includes respective points on the line segment l to be imaged of the central line of the synthetic aperture length L, and the received signal necessary to reproduce the point l.sub.k to be imaged on the line segment l to be imaged includes the value of the received signal on the hyperbolic curve designated by the dotted chain line in FIG. 3. The synthetic aperture length L at this time is defined corresponding to the position of the longest distance in the z-axis direction to be imaged in the x-z plane. The range of the definition of the hyperbolic curve is determined by the spread of the ultrasonic wave beam of the transceiver, being denoted by the broken line in FIG. 3. In other words, in order to image the area of the length of the synthetic aperture length L in the scanning direction, the received signal group including all the scanning points in the scanning range of 2L being twice the synthetic aperture length becomes necessary. In FIG. 3, the received signal group necessary to image the area AR1 to be imaged of the width L in the x-axis direction necessitates all the received signal group in the scanning range SC1 of the length 2L, and the received signal group necessary to image the area AR2 to be imaged of the width L similarly necessitates all the received signal group in the scanning range SC2 of the length 2L.
In case of imaging the area to be imaged in the wide range according to this type method, it is necessary to once A/D-convert the received signal train at the respective scanning points, store them into a memory, input the received signal train at every scanning point into a two-dimensional memory being formed to be the two-dimensional configuration corresponding to the scanning point in one direction and to the time, namely to the distance in the z-axis direction, sequentially pick up the signal value of the received signal train at the respective scanning points determined by the TOF locus line corresponding to the points to be imaged, and add them, since the TOF locus line denoted by the hyperbolic curve for reproducing the image has a different function form from that of the points to be reproduced having the different value with respect to the z-axis direction (the direction perpendicular to the scanning direction). This processing operation is repeatedly executed for all the points to be imaged.
Since the prior-art method of imaging an object by ultrasonic or electromagnetic waves is constructed as described above, the area to be imaged is sequentially updated, and the method requires a large capacity memory for storing the volumious received signal group and extremely long image reproducing time unless considerably effort is put into devising storage for the received signal train of scanning points and for processing the image reproduction, in the case of non-destructive inspection of a pipe etc. wherein sequential updating of the area to be imaged and imaging of a wide range area must be performed. Further, the reproduced image results in a drawback of attenuation due to the space propagation characteristic of ultrasonic or electromagnetic waves, i.e., beam spreading and propagation distance, and if no correction is executed, the physical information such as the scattering coefficient of the object cannot be suitably obtained.